Cercarial dermatitis, also known as swimmer's itch or clam-digger's itch, is caused by the larvae of blood flukes that are parasites of birds or mammals. When these larvae, called cercariae, penetrate human skin, they trigger an allergic reaction within 10-15 hours that takes about a week to heal. Unable to mature into adults, the larvae then die on the skin. The gravity of an outbreak depends on how humans and birds or mammals come in contract with the aquatic environment, but people engaged in water activities, such as farmers, fishermen, and agricultural workers, are most likely to be affected.